As to communications terminal devices such as mobile phone terminals, there has been known TDMA system, in which time slots can be set for an idle state, an operation to receive a signal from a base station or an operation to send a signal to the base station. Incidentally, TDMA is an abbreviation of Time-Division Multiple Access. As the TDMA systems have been known GSM system and GMSK system, which use only phase modulation, provided that GSM is an abbreviation of Global System for Mobile Communication, and GMSK is an abbreviation of Gaussian minimum Shift Keying. In contrast with GSM and GMSK systems, systems which improve a communication data transfer rate have been known. As one of such improvement systems, EDGE system using amplitude modulation in parallel with phase modulation has been attracting attention recently. Incidentally, EDGE is an abbreviation of Enhanced Data for GSM Evolution or Enhanced Data for GPRS, and GPRS is an abbreviation of General Packet Radio Service.
As a method to materialize the EDGE system has been known a polar loop system including: separating a transmitter signal, which must be sent out, into phase and amplitude components; then performing respective feedback controls with phase and amplitude control loops; and combining phase and amplitude components gained after the feedback controls by means of an amplifier.
A polar loop transmitter having a phase control loop and an amplitude control loop and supporting an EDGE transmitter function is described in the Non-patent Document presented by Earl McCune, “High-Efficiency, Multi-Mode, Multi-Band Terminal Power Amplifiers”, IEEE microwave magazine, March 2005, PP. 44-45. According to the document, it is described that the power efficiency is a key market issue for mobile phones, and in a polar loop system, an RF power amplifier operating in a condition near saturation offers the advantage of a better power efficiency. Also, it is described that an additional advantage of the polar loop system posed by an operation of an RF power amplifier in such near-saturation condition is low-noise characteristics.
In contrast, specifications of a digital interface between an RF IC and a baseband LSI are stated in the Non-patent Document presented by Andrew Fogg, “DigRF BASEBAND/RF DIGITAL INTERFACE SPECIFICATION”, Logical, Electrical and Timing Characteristics EGPRS Version Digital Interface Working Group Rapporteur Andrew Fogg, TTPCom Version 1.12 http://146.101.169.51/DigRF Standard v112.pdf (according to search on Oct. 5, 2006).